Why do you need to replace your pump? Folllowing 4 Yanmar forums over a 5 year period, I have never heard of anyone needing to replace one. They are rebuildable, much cheaper.
Does the tractor have a loader on it? That is the only time I have seen those break like that. Usually the loader hoses are uncoupled with the tractor running or the tractor is started with the loader hoses uncoupled. The oil has no where to go and cracks the pump case or twists off the pump shaft.
May be someone will have a used pump they can sell you. I'am sure a new pump will b high. I guess a used 1 will depend on how many F18's are over here.